Pleurobema sintoxia
round pigtoe

The round pigtoe was historically found in the upper Mississippi drainages and the St. Lawrence drainage. Generally it is found from western New York and Ontario west through Wisconsin and Minnesota to the southeastern portion of South Dakota, south to Oklahoma, Arkansas, Mississippi and Alabama. Its eastern range includes western Virginia, West Virginia, and western Pennsylvania.

In Michigan P. sintoxia is found in mid-Michigan drainages, mainly from the Muskegon and Saginaw River systems on through the southern river systems. Specimens have also been collected in the Sturgeon River in the upper peninsula. (Burch, 1975; COSEWIC, 2004; Cummings and Mayer, 1992)

The round pigtoe is generally widespread from small to large rivers. Substrates it inhabits include mud, sand and gravel with moderate flows.

In Lake St. Clair it is found in nearshore areas less than 1 m with firm, sandy substrates. (COSEWIC, 2004; Watters, 1995; Zanatta et al., 2002)

The round pigtoe is up to 10.2 cm (4 inches) long , and is round and compressed in headwaters, triangular and inflated in rivers. The shell is usually fairly thick. The anterior end is broadly rounded, the posterior end rounded to bluntly pointed. The dorsal margin is straight to slightly curved and the ventral margin is gently curved.

Umbos are low, being raised only slightly above the hinge line, and can be centered on the shell or more anterior. The beak sculpture with two elevated ridges on the umbo or posterior ridge.

The periostracum (outer shell layer) is green-brown, red-brown or light brown with green rays. Older specimens tend to be more brown and lack rays.

On the inner shell, the left valve has two pseudocardinal teeth, which are rough, large, and grooved. The anterior tooth is about one third the size of the posterior tooth. The two lateral teeth are straight, short, and somewhat striated. The right valve has one large, erect, notched and grooved pseudocardinal tooth. The one lateral tooth has high, broad and striated.

The beak cavity is shallow to moderately deep. Although the nacre is white, occasionally it is has a pink or salmon tint and is iridescent at the posterior end.

In Michigan, this species is most difficult to distinguish from the headwater forms of the Wabash pigtoe, and is often only distinguished by looking at the teeth on the inner part of the valves of the shells. The Wabash pigtoe has a deeper beak cavity. When present in females, glochidial packets from the Wabash pigtoe also tend to be orangish while the round pigtoe has white glochidial packets. Other similar species include the hickorynut and the round hickorynut, which are generally more inflated. The hickorynut is more slightly more oval in shape. The round hickorynut is more rounded and generally lighter at the posterior end. (Cummings and Mayer, 1992; Oesch, 1984; Watters, 1995)

Fertilized eggs are brooded in the marsupia (water tubes) up to three months, where they develop into larvae, called glochidia. The glochidia are then released into the water where they must attach to the gill filaments and/or general body surface of the host fish. After attachment, epithelial tissue from the host fish grows over and encapsulates a glochidium, usually within a few hours. The glochidia then metamorphoses into a juvenile mussel within a few days or weeks. After metamorphosis, the juvenile is sloughed off as a free-living organism. Juveniles are found in the substrate where they develop into adults. (Arey, 1921; Lefevre and Curtis, 1910)

The age of mussels can be determined by looking at annual rings on the shell. However, no demographic data on this species has been recorded.

Mussels in general are rather sedentary, although they may move in response to changing water levels and conditions. Although not thoroughly documented, the mussels may vertically migrate to release glochidia and spawn. (Oesch, 1984; Watters, 1995)

The middle lobe of the mantle edge has most of a bivalve's sensory organs. Paired statocysts, which are fluid filled chambers with a solid granule or pellet (a statolity) are in the mussel's foot. The statocysts help the mussel with georeception, or orientation.

Mussels are heterothermic, and therefore are sensitive and responsive to temperature.

Unionids in general may have some form of chemical reception to recognize fish hosts. How the round pigote attracts its fish host is unknown. However, white glochidial packets that are released resemble invertebrates that fish would eat.

Glochidia respond to both touch, light and some chemical cues. In general, when touched or a fluid is introduced, they will respond by clamping shut. (Arey, 1921; Brusca and Brusca, 2003; Watters, 1995)

In general, unionids are filter feeders. The mussels use cilia to pump water into the incurrent siphon where food is caught in a mucus lining in the demibranchs. Particles are sorted by the labial palps and then directed to the mouth. Mussels have been cultured on algae, but they may also ingest bacteria, protozoans and other organic particles.

The parasitic glochidial stage absorbs blood and nutrients from hosts after attachment. Mantle cells within the glochidia feed off of the host’s tissue through phagocytocis. (Arey, 1921; Meglitsch and Schram, 1991; Watters, 1995)

Unionids in general are preyed upon by muskrats, raccoons, minks, otters, and some birds. Juveniles are probably also fed upon by freshwater drum, sheepshead, lake sturgeon, spotted suckers, redhorses, and pumpkinseeds.

Unionid mortality and reproduction is affected by unionicolid mites and monogenic trematodes feeding on gill and mantle tissue. Parasitic chironomid larvae may destroy up to half the mussel gill. (Cummings and Mayer, 1992; Watters, 1995)

Fish hosts are determined by looking at both lab metamorphosis and natural infestations. Looking at both is necessary, as lab transformations from glochidia to juvenile may occur, but the mussel may not actually infect a particular species in a natural situation. Natural infestations may also be found, but glochidia will attach to almost any fish, including those that are not suitable hosts. Lab transformations involve isolating one particular fish species and introducing glochidia either into the fish tank or directly inoculating the fish gills with glochidia. Tanks are monitored and if juveniles are later found the fish species is considered a suitable host.

In lab trials, Pleurobema sintoxia glochidia metamorphosed on the bluntnose minnow, northern redbelly dace, southern redbelly dace and spotfin shiner. (Hove et al., 1995; Hove, 1995)

There are no significant negative impacts of mussels on humans.

Mussels are ecological indicators. Their presence in a water body usually indicates good water quality.

The round pigtoe is listed as state endangered in Iowa, threatened in Minnesota, and special concern in Michigan. (Hove, 2004)

Pleurobema sintoxia is synonomous with Pleurobema coccineum.